1. The Field of the Invention
The present invention relates generally to the field of optical transceivers and their use. More particularly, exemplary embodiments of the invention are concerned with optical transceivers that include circuits and components which implement a desired preemphasis in transmitted optical signals.
2. Related Technology
As fiber optic transmission systems are pushed to higher data rates and longer transmission distances, the performance of those systems is often limited by one form or another of optical dispersion, which typically occurs as a result of the different respective velocities of the components of an optical signal. More particularly, such differences in respective velocities of the components, sometimes referred to as the “velocity spread,” result in the spreading of the optical pulses over time. Because optical data transmission systems rely for their functionality on the transmission and detection of pulses that correspond either to a digital “1” (high power) or digital “0” (low power), spreading of these pulses is a matter of concern.
In particular, the velocity spread means that the pulses that make up the 0s and 1s tend to spread into, or overlap, one another, leading to a condition sometimes referred to as inter-symbol interference (“ISI”). ISI is undesirable because as the extent to which the optical components spread into each other increases, it becomes increasingly difficult, if not impossible, to reliably identify any clear distinction in the power level of a 0 or a 1. This phenomenon is sometimes illustrated graphically in the form of a “closed” optical eye pattern. Because most detection systems rely for their effectiveness on the ability to distinguish between a “1” and “0,” with a simple thresholding circuit, the closed, or impaired, eye pattern that results from ISI represents a significant impairment to the operability and usefulness of an optical system.
The problem of ISI has been addressed in various non-optical communication links, such as copper-based high speed electronic links, by implementing compensation in either, or both of, the transmitter and receiver of the transmission system. On the receive side of the system, passive equalization circuits or more sophisticated adaptive electronic equalization have been used. The latter arrangement is often referred to as electronic dispersion compensation (“EDC”).
As suggested above, compensation for ISI can also be implemented on the transmitter side of the non-optical system. Compensating for ISI by modifying the transmitter signal is sometimes referred to as “transmitter preemphasis” since this type of method most commonly involves boosting, in some manner, the high frequency content of the transmitted electrical data signal in an attempt to overcome the typical. overall high frequency rolloff of the channel response. Less commonly, other transmitter preemphasis techniques are concerned with “deemphasis,” which generally involves deemphasizing low frequencies. In any case, the basic principle is the same, namely, attempting to compensate for the channel frequency response by generating a transmit signal close to the inverse of that frequency response. As discussed elsewhere herein however, the effectiveness of such techniques in copper-based, and other non-optical, links is largely due to the fact that the frequency response of copper and similar media is highly predictable.
The use of electronic equalizers to implement ISI compensation through the use of an EDC mechanism is well known in radio transmission, copper-based high speed electronic links, and disk drive read circuits. More recently, EDC has been used to a much more limited extent in selected optical systems to extend the distances over which high speed links based on electro-absorption modulated lasers (“EML”) can operate. Although such systems are often susceptible to wavelength chirp, or shifting of the center wavelength of the EML, EDC techniques can, in some cases, improve the performance and effective transmission distances of such systems.
As another example, EDC techniques have been demonstrated, in some cases, to contribute to improvements in the performance and effective transmission distances of data transmission networks that employ legacy multimode fiber. For the most common grade of presently installed multimode fiber, conventional transceivers can generally not achieve transmission distances beyond 100 m, whereas the most interesting use of these links require transmission distances of at least 220 m with a strong preference for 300 m. In the case of multimode fiber, link distances are limited by modal dispersion, that is, the differences in the effective velocity of the different fiber modes caused by imperfections in the index profiles of the fibers. Depending on the degree of these imperfections, EDC techniques can often be used to achieve the desired distance of 300 m. However, it appears that an important fraction of these fibers may have imperfections that are so great that they cannot be equalized with practical EDC techniques.
This is a matter of significant concern since although many enterprises demand increasingly high levels of performance, such as 10 Gb/s or higher data rates over links of up to 300 m, those same enterprises are often unwilling and/or unable to invest in replacement of an existing legacy system with a new fiber network infrastructure that can support such data rates.